Method for inhibiting polymerization during the recovery and purification of unsaturated mononitriles

ABSTRACT

Economical processes are disclosed for recovery and refining of valuable nitrogen-containing organic compounds formed by catalytic oxidation of least one feed compound selected from the group consisting of propane, propylene, isobutane and isobutylene in the presence of ammonia to produce a gaseous. Processes of the invention include quenching the gaseous reactor effluent with an aqueous quench liquid; forming an aqueous solution comprising the corresponding unsaturated mononitrile, hydrogen cyanide and other organic co-products; and using an integrated sequence of distillations and phase separations to recover for recycle of a useful aqueous liquid, and obtain the desired nitrogen-containing products. According to the invention aqueous solutions are fractionated in an integrated system of multi-stage columns while an effective polymerization inhibiting amount of at least one member of a preselected class of p-phenylenediamine compounds is maintained therein.

FIELD OF THE INVENTION

[0001] The field of this invention relates to continuous processes forrecovery and purification of organic values from hot gaseous mixtureswhich are obtained by catalytic ammoxidation of a light hydrocarboncompounds. More particularly, this invention relates to recovery andrefining of valuable nitrogen-containing organic compounds formed bycatalytic oxidation of least one feed compound selected from the groupconsisting of propane, propylene, isobutane and isobutylene in thepresence of ammonia to produce a gaseous reactor effluent containing thecorresponding unsaturated mononitrile. Processes of the inventioninclude quenching the gaseous reactor effluent with an aqueous quenchliquid; forming an aqueous solution comprising the correspondingunsaturated mononitrile, hydrogen cyanide and other organic co-products;and using an integrated sequence of distillations and phase separationsto recover for recycle of a useful aqueous liquid, and obtain valuablenitrogen-containing organic compounds and hydrogen cyanide products.Beneficially, according to the invention aqueous solutions arefractionated in an integrated system of multi-stage columns while aneffective polymerization inhibiting amount of at least one member of apreselected class of p-phenylenediamine compounds is maintained therein.

BACKGROUND OF THE INVENTION

[0002] As is well known, most of the commercial acrylonitrile isproduced with the Sohio Process from propylene by heterogeneouscatalytic ammoxidation of propylene in the vapor phase with ammonia, airand steam. For exmple see U.S. Pat. No. 3,222,422 in the name of L. A.Cohen; U.S. Pat. Nos. 3,278,642 and 3,346,520 both in the name of L.Lee; U.S. Pat. No. 3,442,981 in the name of O. L. Stafford, D. V. Wingand D. E. Stolsmark; and U.S. Pat. No. 3,509,238 in the name of N. E.Aubery and M. B. Jastrzebeski.

[0003] In a commercial acrylonitrile system utilizing this process, thereactor feeds are propylene, ammonia and compressed air. The propyleneand ammonia are vaporized, then combined with the air and fed to afluidized bed catalytic reactor. Precise ratios of the three feeds aremaintained for optimum yield. The catalyst in the reactor vessel is inthe form of particles, which are maintained in a turbulent fluid stateby the velocity of gaseous flow through the bed.

[0004] Propylene, ammonia and oxygen mix together in the reactor andoxidation of propylene in the presence of ammonia takes place on thesurface of the fluidized catalyst. A set of complex exothermic reactionstakes place, thereby forming the following products: acrylonitrile,hydrogen cyanide, carbon dioxide, carbon monoxide, acetonitrile,acrolein, acrylic acid, water, other higher nitrites, aldehydes,ketones, acetic acid and a number of miscellaneous unknown organiccompounds. Conversion of the three feeds is less than 100 percent, thusunreacted propylene, ammonia, oxygen and nitrogen are contained in thereactor effluent gas. The source of propylene typically contains a smallamount of propane and some heavier hydrocarbon compounds which most ofwhich are purged from the process unreacted. A portion of the heat ofthe exothermic reaction is removed by sets of steam coils which generateand superheat waste steam at approximately 600 psig for process usessuch as heat input for 9distillations in the products recovery andpurification section of the process. Reactor effluent gas passes throughcyclones, which remove catalyst fines from the gas. The gas is thenfurther cooled in a reactor effluent cooler, which is comprised of ashell and tube exchanger using boiler feed-water as the cooling source.

[0005] As the gas leaves the reactor effluent cooler, it then enters aquench column. The quench column cools the reactor effluent bycontacting it with a recirculating water stream. Most of the water vaporand small amounts of organic vapors in the reactor effluent arecondensed in the quench column. The quench column bottoms are cooled andcirculated back to the quench column. The quench column can containinternal trays or packing to provide intimate contact of upflowing gaswith downflowing water. Sulfuric acid is injected into the recirculatingquench water to neutralize unreacted ammonia in the reactor effluent.The excess quench water is roughly equal to the amount of water producedby the reactor and is fed to the wastewater column where acrylonitrileand hydrogen cyanide are recovered. Wastewater column bottoms are cooledand neutralized, mixed with other plant waste streams, clarified andinjected into the wastewater injection well. The quench column effluentgas is then directed to an absorber where chilled water is used toobtain an aqueous solution of acrylonitrile, hydrogen cyanide and otherorganics from the gas.

[0006] The aqueous solution from the absorber is fed to a recoverycolumn where acrylonitrile and hydrogen cyanide are taken overhead. Aportion of the bottoms from the recovery column is cooled and recycledto the absorption step. This recycle contains both inorganic and organiccompounds in the form of monomers, oligomers, prepolymers, and polymersin various combinations. Acrylonitrile, hydrogen cyanide and optionallyacetonitrile products are then purified using a series of distillationsand phase separations. A first column (heads column) removes hydrogencyanide, and at last column (acrylonitrile product column) takes a pureacrylonitrile monomer product from a side-draw near the top of thecolumn. High-boiling organic compounds are rejected from the productcolumn bottoms.

[0007] Acrylonitrile can polymerize in the quench column. Morespecifically, as the reactor effluent gas is passed through the quenchcolumn, a portion of the acrylonitrile contained in the gas polymerizesand is absorbed into the recirculating quench water. The amount ofacrylonitrile that polymerizes in the quench column represents anundesirable net product loss for the acrylonitrile plant. For example,in an uninhibited quench column, between about 2 to 5 percent of thetotal acrylonitrile produced by the reactor is lost due topolymerization in the quench column.

[0008] Several methods are known to reduce losses of acrylonitrile bypolymerization and other side reactions, which involve treating therecirculating quench water. For example see U.S. Pat. Nos. 3,691,226;4,720,566; 5,869,730; 5,895,822 and 6,238,574, which patents areincorporated herein by reference.

[0009] U.S. Pat. No. 4,720,566, in the name of John F. Martin, describesmethods and compositions for inhibiting acrylonitrile polymerization inquench columns of systems producing acrylonitrile with a combination of(a) a hydroxylamine having two alkyl groups, and (b) apara-phenylenediamine with a substituent phenyl group or unsubstitutedpara-phenylenediamine.

[0010] Unfortunately, under operating conditions, acrylonitrile can alsopolymerize in the recovery and purification sections to from soliddeposits which interfere with operation of equipment, contribute to anundesirable net production loss and reduction in production rates, andwith time lead to costly shutdowns.

[0011] Such polymers, oligomers, and prepolymers, in variouscombinations, foul heat exchange surfaces of the heat exchangers used tomaintain operating conditions for separation in the distillation columnsand other process equipment. Fouled heat exchange surfaces reduce thecoefficient of heat transfer thereby increasing the amount of heattransfer medium which must be used to realize the required amount ofheating and/or cooling obtained on clean surfaces. Eventually, the heatexchanger must be manually cleaned with potential exposure of personnelto hazardous chemicals.

[0012] It is therefore a general object of the present invention toprovide an improved process which overcomes the aforesaid problem ofprior art methods for to recovery and refining of valuablenitrogen-containing organic compounds formed by catalytic oxidation ofleast one feed compound selected from the group consisting of propane,propylene, isobutane and isobutylene in the presence of ammonia.

[0013] Improved processes would utilize a preselected class ofpolymerization inhibiting compositions effective under operatingconditions during fractional distillations of aqueous solutionscomprising the unsaturated mononitrile products.

[0014] Advantageously, members of such a class of inhibitingcompositions would be effectively separated by the fractionaldistillation of the purified products.

[0015] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and appendedclaims.

SUMMARY OF THE INVENTION

[0016] Economical processes are disclosed for recovery of valuablenitrogen-containing organic compounds formed by catalytic ammoxidationof propane, propylene, isobutane, and/or isobutylene with ammonia and agaseous source of dioxygen and steam. Nitrogen-containing organiccompounds produced in the catalytic oxidation reactions are recoveredfrom product gaseous stream as a aqueous solution. The aqueous solutioncontaining the unsaturated mononitrile is transferred to a recovery andpurification section where the unsaturated mononitrile is recovered andpurified in at least a first distillation column and at least oneproduct distillation columns wherein high-boiling impurities areseparated from the unsaturated mononitrile by distillation.

[0017] Under operating conditions, unsaturated mononitrile, e.g.,acrylonitrile or methacrylonitrile, and other organic compounds present,can polymerize in the recovery and purification sections to from soliddeposits which interfere with operation of equipment, contribute to anundesirable net production loss and reduction in production rates, andwith time lead to costly shutdowns.

[0018] Processes of this invention comprise: (a) reacting at least onefeed compound selected from the group consisting of propane, propylene,isobutane and isobutylene, with ammonia and a source of dioxygen in thepresence of a catalyst a reactor to produce a reactor effluentcontaining the corresponding unsaturated mononitrile; (b) transferringthe reactor effluent containing the unsaturated mononitrile to aquench/absorption section wherein the reactor effluent containing theunsaturated mononitrile is contacted with at least a first aqueousstream to cool the reactor effluent, and thereafter the cool effluent iscontacted with at least a second aqueous stream in an absorption columnto separate and recover the unsaturated mononitrile in an aqueoussolution; (c) transferring the aqueous solution containing theunsaturated mononitrile to a recovery and purification section where theunsaturated mononitrile is recovered and purified in at least a firstdistillation column and a second distillation column; and (d)maintaining within one or more of the distillation columns an effectivepolymerization inhibiting amount of at least one phenylenediaminecompound represented by the formula:

[0019] wherein Ph is a phenylene group, and R₁ and R₂ are the same ordifferent and are hydroxyl free organic moieties having about 2 to about10 carbon atoms with the proviso that neither R₁ nor R₂ is a phenylgroup.

[0020] In a preferred class of phenylenediamine compounds the organicmoieties (R₁ and R₂) are members of a group consisting of ethyl, propyl,butyl, phenyl, hexyl, heptyl, octyl nonyl and decyl hydrocarbon groups,particularly straight and branched-chain hydrocarbon groups.Advantageously, this class consists of N,N′-dialkyl-p-phenylenediaminecompounds. More preferably at least one of the compounds is a member ofa group consisting of N,N′-di-sec-propyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine,N,N′-di-isobutyl-p-phenylenediamine, andN,N′-di-tert-butyl-p-phenylenediamine. Most preferably thep-phenylenediamine is N,N′-di-sec-butyl-p-phenylenediamine for bestresults.

[0021] In preferred embodiments of the invention, the effectivepolymerization inhibiting amount is no less than about 5 parts permillion parts of unsaturated mononitrile present in the aqueoussolution. Generally, the effective polymerization inhibiting amount isin a range upward from about 10 to about 10,000 parts per million partsof acrylonitrile in the aqueous solution. Preferably in a range fromabout 50 to about 1000 parts per million parts of acrylonitrile theaqueous solution. Most preferably in a range from about 75 to about 750parts per million parts of acrylonitrile the aqueous solution.

[0022] Processes of the invention preferably include admixing of aliquid source of the phenylenediamine compound with an aqueous solutioncontaining the unsaturated mononitrile which is being transferred intothe first distillation column and/or one or more product distillationcolumns. Preferably the phenylenediamine compound is in the liquid stateat conditions from ambient up to about 60° C., however the liquid sourceof the phenylenediamine compound may also be a solution of thephenylenediamine compound in the unsaturated mononitrile, water, or acombination thereof.

[0023] In preferred embodiments of the invention, the unsaturatedmononitrile is acrylonitrile or methacrylonitrile, and the feed compoundis a corresponding olefin selected from the group consisting ofpropylene and isobutylene. In other preferred embodiments of theinvention, the unsaturated mononitrile is acrylonitrile, and the feedcompounds are members of a group consisting of propane and propylene.

[0024] One aspect of the invention provides a process for recovery ofvaluable nitrogen-containing organic compounds formed by catalyticammoxidation of propane, propylene or isobutylene with ammonia and agaseous source of dioxygen which process comprises: providing an aqueoussolution comprising acrylonitrile or methacrylonitrile, hydrogen cyanideand other organic co-products; fractionating the aqueous solution as bydistillation in at least a first multi-stage column and a secondmulti-stage column; and maintaining within the columns an effectivepolymerization inhibiting amount of at least one compound represented bythe formula I, wherein Ph is a phenylene group, and R₁ and R₂ are thesame or different and are hydroxyl free organic moieties having about 2to about 10 carbon atoms with the proviso that neither R₁ nor R₂ is aphenyl group. Advantageously, the effective polymerization inhibitingamount is in a range upward from about 50 to about 1000 parts permillion parts of unsaturated mononitrile present in the aqueoussolution.

[0025] An aspect of special significance is that the feed compounds aremembers of a group consisting of propane and propylene.

[0026] Another aspect of the invention provides a process for recoveryof valuable nitrogen-containing organic compounds formed by catalyticammoxidation of propylene and/or propane with ammonia and a gaseoussource of dioxygen which process comprises: providing an aqueoussolution comprising acrylonitrile, hydrogen cyanide and other organicco-products of an ammoxidation reaction of propylene and/or propane withammonia and a gaseous source of dioxygen; fractionating the aqueoussolution as by distillation in a multi-stage column to obtain a highboiling fraction comprising a major amount of the acrylonitrile in theaqueous solution and a low boiling fraction comprising a major amount ofthe hydrogen cyanide in the aqueous solution; and maintaining within thecolumn an effective polymerization inhibiting amount of at least onephenylenediamine compound represented by the above formula I: wherein Phis a phenylene group, and R₁ and R₂ are the same or different and arehydroxyl free organic moieties having about 2 to about 10 carbon atomswith the proviso that neither R₁ nor R₂ is a phenyl group.

[0027] Yet another aspect of the invention provides a process forrecovery of valuable nitrogen-containing organic compounds formed bycatalytic ammoxidation of propylene or propane with ammonia and agaseous source of dioxygen which process comprises: providing an aqueoussolution comprising acrylonitrile and high-boiling organic compounds;fractionating the aqueous solution as by distillation in a multi-stagecolumn to obtain a high boiling fraction comprising a high boilingfraction comprising essentially all the organic compounds boiling aboveabout 100° C., a sidedraw product stream comprising at least 99 percentby weight of acrylonitrile, and a low boiling fraction substantiallyfree of the high-boiling organic compounds; and maintaining within thecolumn an effective polymerization inhibiting amount of at least onecompound represented by the above formula I: wherein Ph is a phenylenegroup, and R₁ and R₂ are the same or different and are hydroxyl freeorganic moieties having about 2 to about 10 carbon atoms with theproviso that neither R₁ nor R₂ is a phenyl group.

[0028] A preferred aspect of the invention provides a process forrecovery of valuable nitrogen-containing organic compounds formed bycatalytic ammoxidation of isobutylene or isobutane with ammonia and agaseous source of dioxygen which process comprises: providing an aqueoussolution comprising methacrylonitrile, hydrogen cyanide and otherorganic co-products of an ammoxidation reaction of isobutylene and/orisobutane with ammonia and a gaseous source of dioxygen; fractionatingthe aqueous solution as by distillation in a multi-stage column toobtain a high boiling fraction comprising a major amount of themethacrylonitrile in the aqueous solution and a low boiling fractioncomprising a major amount of the hydrogen cyanide in the aqueoussolution; and maintaining within the column an effective polymerizationinhibiting amount of at least one compound represented by the aboveformula I: wherein Ph is a phenylene group, and R₁ and R₂ are the sameor different and are hydroxyl free organic moieties having about 2 toabout 10 carbon atoms with the proviso that neither R₁ nor R₂ is aphenyl group.

[0029] Another preferred aspect of the invention provides a process forrecovery of valuable nitrogen-containing organic compounds which processcomprises: forming a gaseous reaction effluent by ammoxidation ofpropylene with ammonia and a gaseous source of dioxygen in the presenceof a heterogeneous catalyst comprising cobalt; contacting the gaseousreaction effluent with an aqueous liquid to obtain an aqueous solutioncomprising acrylonitrile, hydrogen cyanide and other organicco-products; fractionating the aqueous solution as by distillation in amulti-stage column to obtain a high boiling fraction comprising a majoramount of the acrylonitrile in the aqueous solution and a low boilingfraction comprising a major amount of the hydrogen cyanide in theaqueous solution; and maintaining within the column an effectivepolymerization inhibiting amount of at least one compound represented bythe above formula I: wherein Ph is a phenylene group, and R₁ and R₂ arethe same or different and are hydroxyl free organic moieties havingabout 2 to about 10 carbon atoms with the proviso that neither R₁ nor R₂is a phenyl group.

[0030] The total amount of p-phenylenediamine compounds of thepreselected class used in the compositions and methods of the presentinvention as polymerization inhibitor is that amount which is sufficientof effect inhibition of polymerization of the unsaturated aliphaticnitrile product and will, of course, vary according to the particularconditions under which they are used. Where process stream aremaintained at higher temperatures and/or longer durations, larger amuntsare generaly required. Preferably, during one or more of theextractions, distillations, and phase separations for recovery andpurification, the total amount of of p-phenylenediamine compounds of thepreselected class is no less than about 5 parts per million parts ofunsaturated aliphatic nitrite present in the aqueous solution.Typically, the effective polymerization inhibiting amount is in a rangeupward from about 10 to about 10,000 parts per million parts ofunsaturated aliphatic nitrite e in the aqueous solution. More preferablyin a range from about 50 to about 1000 parts per million parts ofunsaturated aliphatic nitrite the aqueous solution. Most preferably in arange from about 75 to about 750 parts per million parts of unsaturatedaliphatic nitrite the aqueous solution.

[0031] For a more complete understanding of the present invention,reference should now be made to the embodiments illustrated in greaterdetail in the accompanying drawing and described below by way ofexamples of the invention.

BRIEF DESCRIPTION OF THE FIGURE

[0032] The appended claims set forth those novel features whichcharacterize the present invention. The present invention itself, aswell as advantages thereof, may best be understood, however, byreference to the following brief description of preferred embodimentstaken in conjunction with the annexed drawing, in which:

[0033] The FIGURE is a schematic diagram depicting a preferred methodfor operating the process of this invention in the continuous mode beingarranged to provide an integrated sequence of extractions,distillations, and phase separations for recovery and purification, andto maintain suitable operating conditions in accordance with the presentinvention, thereby obtaining valuable nitrogen-containing organiccompounds.

BRIEF DESCRIPTION OF THE INVENTION

[0034] Processes of this invention are particularly suitable for use inrecovery and refining of an unsaturated aliphatic nitrite product, suchas acrylonitrile or methacrylonitrile, from a gaseous mixture produced,generally, by gas-phase catalytic oxidation and/or thermal oxidation ofolefin and ammonia with a source of dioxygen, typically comprisingcompressed air. The well known and most widely used commercial processesfor production of acrylonitrile by ammoxidation of propylene withammonia is the Sohio Process.

[0035] As is well known, performance of the oxidation catalysts is animportant factor, perhaps the most significant factor, in the economicsof this and other oxidation processes. Catalyst performance is measuredby activity, i.e., conversion of reactants, selectivity, i.e. conversionof reactant to desired product, rate of production of desired productper unit of reactor volume per unit of time, and catalyst life, i.e.effective time on-stream before significant loss of activity orselectivity.

[0036] The practice of the process of the present invention is notdependent upon any specific ammoxidation fluid bed catalyst. Suitablecatalysts which are more selective for the ammoxidation of propylene andisobutylene can be prepared from bismuth, cobalt, iron, nickel, tinsalts, and molybdic, molybdic phosphoric, and molybdic silicic acids.Other components, such as tungsten, copper, tellurium, and arsenicoxides, have been incorporated to increase low temperature activity andproductivity.

[0037] Factors upon which catalyst performance depends includecomposition, the methods of preparation, support, and calcinationconditions. Other key properties include, In addition to chemicalperformance requirements, other key properties include surface area,porosity, density, pore size distribution, hardness, strength, andresistance to mechanical attrition.

[0038] Patents claiming specific catalysts and processes for their usein the manufacture of acrylonitrile and methacrylonitrile by theammoxidation of propylene and isobutylene, respectively, include U.S.Pat. Nos. 2,481,826; 2,904,580; 3,044,966; 3,050,546; 3,197,419;3,198,750; 3,200,084; 3,230,246, 3,248,340 and 3,352,764 which patentsare incorporated herein by reference.

[0039] Feeds for the ammoxidation combine to contain a mixture ofpropylene, air, ammonia, and optionally an supplementary source ofdioxygen. Beneficially, feed compositions range up to about 9 percentpropylene on molar basis. Gaseous mixtures in the ammoxidation reactorsare kept too low in oxygen to be flammable during normal operation.Reactor start-up and shutdown procedures are likewise designed to avoidflammable feed mixtures.

[0040] Effluent from the ammoxidation reactor is cooled in a quenchtower with an acidified water stream by counter-current contact. Gasesfrom the quench tower are transferred into the bottom of an absorberwhere acrylonitrile, acetonitrile and other relatively soluble gases areabsorbed. The non-absorbed gases are vented.

[0041] The stream from the bottom of the absorber, known as the richwater stream, is transferred into a recovery column where it isextractively distilled. The recovery column may be any suitablecontacting means in which liquid and vapor are counter-currentlycontacted in a multiplicity of communicating zones or stages. Theoverhead vapors from the recovery column are enriched in acrylonitrile,other components being chiefly water and hydrogen cyanide, andcontaminated with undesirable impurities such as nitrites, compoundswhich have characteristics of nitrites, and precursors thereof. Theoverhead vapors are condensed and collected in a decanter, the liquidundergoes liquid—liquid phase separation, the less dense layer being anorganic phase, the denser lower layer being an aqueous phase. Theorganic phase being chiefly acrylonitrile contaminated with water andhydrogen cyanide, is withdrawn for further purification. The aqueousphase is refluxed to the upper section of the recovery column.

[0042] Embodiments of the present invention can include known treatmentsof process streams. For example, U.S. Pat. No. 3,442,771 disclosed aprocess for removal of trace impurities (e.g. nitrites, peroxides andprecursors thereof) from unsaturated mononitriles (e.g. acrylonitrile)contaminated with water which process uses the addition of an alkalinesolution to the partially condensed azeotrope of the unsaturated nitrileand water where the azeotrope has been obtained as an overhead streamobtained from an extractive distillation column, in particular, therecovery column. The effect of the alkaline solution is to extract thereaction product of the trace impurities into the aqueous phase of theazeotrope leaving the organic phase relatively impurity-free. Theazeotrope is then transferred into a decanter where liquid—liquid phaseseparation occurs. The organic phase containing crude acrylonitrile isthen removed for further purification while the aqueous phase containingthe reaction products is recycled to the recovery column.

[0043] In a preferred embodiment of the present invention one or more ofthe aqueous recycle streams is treated with an acid to reduce the pH.Preferably, the acid may be a mineral acid such as sulfuric or anorganic acid such as acetic, acrylic, formic or glycol, determined bycost considerations, availability, compatibility, metallurgy, etc.

[0044] Under operating conditions, acrylonitrile can polymerize in therecovery and purification sections to from solid deposits whichinterfere with operation of equipment, contribute to an undesirable netproduction loss and reduction in production rates, and with time lead tocostly shutdowns.

[0045] Beneficially, in the practice of processes of the presentinvention, polymerization inhibitor is fed to each unit to maintain aneffective polymerization inhibiting amount of at least one thepreselected p-phenylenediamine compounds and thereby prevent polymerformation and resulting equipment failure.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0046] While this invention is susceptible of embodiment in manydifferent forms, this specification and accompanying drawing discloseonly some specific forms as an example of the use of the invention. Inparticular, preferred embodiments of the invention which includeammoxidation of propylene with ammonia and a source of dioxygen,quenching the gaseous mixture from the ammoxidation with an aqueousquench liquid to obtain an aqueous solution comprising acrylonitrile,hydrogen cyanide, acetonitrile and other organic co-products, and anintegrated sequence of distillations and phase separations to recoverand purify the acrylonitrile product, recover hydrogen cyanide and,optionally, acetonitrile products are illustrated and/or described.

[0047] As embodied and broadly described herein, the process of thepresent invention comprises reacting an olefin selected from the groupconsisting of propylene and isobutylene, ammonia and oxygen in a reactorzone in the presence of a catalyst to produce a reactor effluentcontaining the corresponding unsaturated mononitrile (i.e. acrylonitrileof methacrylonitrile), transferring the reactor effluent containing theunsaturated mononitrile to a quench column wherein the reactor effluentcontaining the unsaturated mononitrile is contacted with at least afirst aqueous stream to cool the reactor effluent, transferring thecooled reactor effluent containing the unsaturated mononitrile to anabsorption column wherein the reactor effluent containing unsaturatedmononitrile is contacted with at least a second aqueous stream toseparate and remove the unsaturated mononitrile as a bottom stream fromthe absorption column, transferring the bottom stream containing theunsaturated mononitrile to a recovery and purification section where theunsaturated mononitrile is recovered and purified, and recycling atleast one aqueous process stream to improve the efficiency of theprocess.

[0048] The invention is not intended to be limited to the embodiments sodescribed, and the scope of the invention will be pointed out in theappended claims.

[0049] Apparatus of this invention is used with certain conventionalcomponents the details of which, although not fully illustrated ordescribed, will be apparent to those having skill in the art and anunderstanding of the necessary function of such components. Examples ofplant ancillaries not illustrated or described include; facilities forpreparation and distribution of polymerization inhibitor and/orsolutions thereof, steam ejector or vacuum pump systems to maintain therequired operating pressures for distillations at mild conditions oftemperature, collection and disposal systems for waste liquids, andemergency vent systems.

[0050] Preferably processes of this invention derive a suitable gaseousmixture from the air oxidation of an olefin selected from the groupconsisting of propylene and isobutylene over a solid, particulatecatalyst in the presence of ammonia. A suitable sources of olefin cancontain up to about 15 percent of the corresponding alkane, typicallyfrom about 2 to about 10 percent, and up to about 5 percent heavierhydrocarbon compounds, and preferably less than about 2 percent.

[0051] For best results the ammoxidation process is carried out in afluid-bed reactor. for best results. Because of the high olefinconversions obtained, a single pass system is typically satisfactory.Approximately stoichiometric quantities of propylene, ammonia, anddioxygen are introduced into a fluidized bed of catalytic particles.Suitable operating conditions include pressures in a range from about 3to about 35 psig (20.7 to 241.4 kPa gage), more preferably from about 5to about 25 psig (34.5 to 172.4 kPa gage). Generally, temperatures arein a range from about 700° to 1000° F. (371° to 538° C.), preferable ina range from about 750° to 950° F. (399° to 510° C.). Heat of reactionis is removed by generation of steam to control the temperature andgenerating steam at temperatures of from about 300° to about 500°Celevated pressure.

[0052] Advantageously, the oxidations are operated at the lowesttemperature consistent with high conversion. Conversion increases withtemperature; the selectivity generally decreases only with largeincreases in temperature. Catalyst life also decreases with increasingtemperatures. Catalysts are designed to give high performance over arange of operating conditions permitting gradual increase of temperatureover the operating life of the catalysts to maintain productivity andselectivity near the initial levels, thus compensating for gradual lossof catalyst activity.

[0053] Because commercial catalysts achieve high conversion of propyleneto acrylonitrile, once-through operation of the ammoxidation reactorwith a residence time of a few seconds is typical. Commerciallyrecoverable quantities of acetonitrile and hydrocyanic acid are optionalco-products.

[0054] Effluent from the ammoxidation reactor is cooled and is scrubbedwith water in a counter-current, absorbing system from which off-gas,consisting chiefly of nitrogen, is vented. Organic products, primarilyacrylonitrile, acetonitrile and HCN, are collected in water to give upto about 8 percent aqueous acrylonitrile and co-products, preferablyfrom about 2.5 to about 7.5 percent, more preferably from about 3 toabout 7 percent of acrylonitrile and co-products, for best results.

[0055] The aqueous solution of acrylonitrile and co-products is treatedin an integrated system of distillation and phase separation steps bywhich organic products are recovered and at least acrylonitrile isrefined. The aqueous solution is sent to the acrylonitrile recoverycolumn, from which an overhead stream containing crude acrylonitrile andHCN is recovered. A liquid side stream from the column is fractionatedin a small column to the remove acetonitrile as a co-product or, moretypically, for disposal by incineration. Water is removed from thebottom of the acrylonitrile recovery column. Condensate from theoverhead stream is separated, and the HCN removed in the overhead of theheads column. The acrylonitrile in the bottoms is further purified inthe product column to obtain fiber-grade acrylonitrile.

[0056] More specifically with reference to the FIGURE, which is aschematic illustration of an integrated distillation means for obtainingvaluable acrylonitrile product and hydrogen cyanide according to apreferred embodiment of the invention, where separation of organiccompounds from the aqueous solution is illustrated as recovery column 30and decantation drum 40; recovery of the hydrogen cyanide is illustratedas lights separation column 50; and purification of acrylonitrileproduct is illustrated as decantation drum 60 and product column 70.

[0057] Generally, during operation of the integrated processes forrecovery and refining at least acrylonitrile from a gaseous mixtureobtainable by catalytic ammoxidation of propylene and ammonia withdioxygen, an aqueous solution which contains the organic products of theammoxidation is obtained by quenching the gaseous effluent from theammoxidation reactor with an aqueous quench liquid.

[0058] The aqueous solution is fed from the quench system orintermediate storage (not shown) through conduit 22 and into the upperpart of recovery column 30. A liquid stream is withdrawn from near thebottom of recovery column 30 through conduit 23, and cooled in exchanger24. A suitable portion of the cooled liquid stream is dispersed into theupper part of recovery column 30 through conduit 25 and the balance ofthe stream is sent to an acetonitrile recovery and/or disposal (notshown). An aqueous stream from the bottom of recovery column 30 isrecycled to the quench system through manifold 31 and conduit 32. Asneeded in order to maintain suitable conditions of separation inrecovery column 30, liquid from the bottom thereof circulates throughmanifold 31 and conduit 33, reboiler 34 and into the column throughconduit 35.

[0059] After disengagement from the top of recovery column 30, a streamof vapors flows into condenser 38 through conduit 36, and together withcondensate into decantation drum 40 through conduit 39. The more-densephase is withdrawn from decantation drum 40 through conduit 28 and sentback into recovery column 30 typically, combined with the aqueous feedsolution. The less-dense phase is transferred from decantation drum 40through conduit 42 and fed into lights separation column 50.

[0060] As needed in order to maintain suitable conditions of separationin separation column 50, liquid from the bottom thereof circulatesthrough manifold 51 conduit 53, reboiler 54 and into the column throughconduit 55. After disengagement from the top of lights separation column50, a stream of vapors flows through conduit 56 and into condensersystem 57. As needed for reflux, a stream of condensate is transferredinto lights separation column 50, through manifold 58 and conduit 59.(Condensate comprising hydrogen cyanide is sent to hydrogen cyanidestorage tanks (not shown) through conduit 46.

[0061] During operation of the separation column in accordance with theinvention, an effective polymerization inhibiting amount of acomposition comprising at least one p-phenylenediamine represented byformula I, preferably a p-phenylenediamine selected from the groupconsisting of N,N′-di-sec-propyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine,N,N′-di-isobutyl-p-phenylenediamine, andN,N′-di-tert-butyl-p-phenylenediamine, is maintained therein.

[0062] Generally, the effective polymerization inhibiting amount is noless than about 5 parts per million parts of unsaturated mononitrilepresent in the aqueous solution. In preferred embodiments of theinvention, the effective polymerization inhibiting amount is in a rangeupward from about 10 to about 10,000 parts per million parts ofacrylonitrile in the aqueous solution. More preferably, effectiveamounts are in a range from about 50 to about 1000 parts per millionparts of acrylonitrile the aqueous solution. Most preferably, effectiveamounts are in a range from about 75 to about 750 parts per millionparts of acrylonitrile the aqueous solution.

[0063] Liquid is withdrawn from the bottom of separation column 50 istransferred into decantation drum 60 through manifold 51 conduit 52. Themore-dense phase is withdrawn from decantation drum 60 through conduit29 and sent back into recovery column 30 typically, combined with theaqueous feed solution. The less-dense phase is transferred fromdecantation drum 60 through conduit 62 and fed into product column 70near the top the column. Pure acrylonitrile monomer product is withdrawnfrom a side-draw through conduit 78 and sent to storage (not shown).From the bottom of product column 70, an aqueous stream comprising heavyorganic compounds is sent through conduit 72 to a waste water disposalsystem (not shown). After disengagement from the top of product column70, a stream of vapors flows into condenser 66 through conduit 76, and,together with condensate, into decantation drum 60 through conduit 68.

[0064] During operation of the product column and/or decantation inaccordance with the invention, an effective polymerization inhibitingamount of a composition comprising at least one p-phenylenediaminerepresented by formula I, preferably a p-phenylenediamine selected fromthe group consisting of N,N′-di-sec-propyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine,N,N′-di-isobutyl-p-phenylenediamine, andN,N′-di-tert-butyl-p-phenylenediamine, is maintained therein.

[0065] Generally, in these operations the effective polymerizationinhibiting amount is no less than about 5 parts per million parts ofunsaturated mononitrile present in the aqueous solution. In preferredembodiments of the invention, the effective polymerization inhibitingamount is in a range upward from about 10 to about 10,000 parts permillion parts of acrylonitrile in the aqueous solution. More preferably,effective amounts are in a range from about 50 to about 1000 parts permillion parts of acrylonitrile the aqueous solution. Most preferably,effective amounts are in a range from about 75 to about 750 parts permillion parts of acrylonitrile the aqueous solution.

[0066] A stream is withdrawn from a side-draw on recovery column 30through conduit 82 and fed into fractionation column 80. A liquid streamfrom the bottom of fractionation column 80 is returned to recoverycolumn 30 below the side-draw through conduit 84. After disengagementfrom the top of fractionation column 80, a stream of vapors flowsthrough conduit 86 and into condenser 90 where condensate comprisingacetonitrile is formed. A portion of the condensed phase is transferredinto of fractionation column 80, as reflux through manifold 92 andconduit 94. Another portion of the condensed phase is transferredthrough conduit 96 into crude acetonitrile storage tanks and/or disposal(not shown).

[0067] Preferred embodiments of the invention recover an acrylonitrileproduct which contains at least 90 percent of the acrylonitrilecontained in the aqueous solution, more preferably about 95 percent ofthe acrylonitrile contained in the aqueous solution is recovered asmonomer-grade product.

EXAMPLES OF THE INVENTION

[0068] The following Examples will serve to illustrate certain specificembodiments of the herein disclosed invention. These Examples shouldnot, however, be construed as limiting the scope of the novel inventionas there are many variations which may be made thereon without departingfrom the spirit of the disclosed invention, as those of skill in the artwill recognize.

[0069] Equipment

[0070] An internally heated cell was used to screen polymerizationinhibitors. A Hach 2100AN turbidimeter connected to a strip chart wasused to detect solution turbidity. The acrylonitrile used was distilledprior to use. A constant temperature bath was used to circulate the hotwater/ethylene glycol solution through the heated cell to maintain thedesired temperature where the turbidity was monitored.

[0071] General Procedure

[0072] Antioxidants were used as received from the supplier.2,2-Azo-bis-isobutyronitrile (AIBN) was the free radical source. Thegeneral method for testing involved preparing a mixture of freshlydistilled acrylonitrile, an inhibitor and AIBN and subjecting thismixture to an elevated temperature while monitoring turbidity.

Examples 1-3

[0073] These examples demonstrate the use ofN,N′-di-sec-butyl-p-phenylenediamine, identified below as DBPAinhibitor.

Example 1

[0074] A mixture of 18.94 grams of acrylonitrile and 0.00241 grams ofDBPA inhibitor were placed in the hot tube and 0.0266 grams of AIBN wasthen added to the mixture. The mixture was heated to 65° C. and theturbidity was monitored. The onset of turbidity occurred after 228minutes. The molar ratio of AIBN to DBPA inhibitor was 15

Example 2

[0075] Example 1 was repeated except that the molar ratio of AIBN toDBPA inhibitor was 22. The onset of turbidity occurred after 150minutes.

Example 3

[0076] Example 1 was repeated except that the molar ratio of AIBN toDBPA inhibitor was 29. The onset of turbidity occurred after 128minutes.

Comparative Examples A-C

[0077] These comparative examples are to illustrate the use of compoundswhich are hydroxy derivatives of benzene having an OH group is attacheddirectly to a benzene ring, i.e., phenols, to inhibit polymerization.

Comparative Example A

[0078] A mixture of 18.97 grams of acrylonitrile and 0.00232 grams ofhydroquinone (p-dihydroxybenzene, M.P. 170.3° C.) were placed in the hottube and 0.0554 grams of AIBN was then added to the mixture. The mixturewas heated to 65° C. and the turbidity was monitored. The onset ofturbidity occurred after 74 minutes. The molar ratio of AIBN toinhibitor was 16.

Comparative Example B

[0079] Comparative Example A was repeated except that the molar ratio ofAIBN to inhibitor was 18. The onset of turbidity occurred after 56minutes.

Comparative Example C

[0080] Comparative Example A was repeated except that the molar ratio ofAIBN to inhibitor was 31. The onset of turbidity occurred after 20minutes.

[0081] These data show that the time to onset of turbity, over range forthe molar ratio of AIBN to inhibitor of 15 to 30, the use ofN,N′-di-sec-butyl-p-phenylenediamine was 3 to 4⅔ times longer than usinghydroquinone.

Comparative Example D

[0082] A mixture of 19.26 grams of acrylonitrile and 0.00087 grams ofhydroquinone and 0.00106 grams of DBPA inhibitor were placed in the hottube and 0.0500 grams of AIBN was then added to the mixture. The mixturewas heated to 65° C. and the turbidity was monitored. The onset ofturbidity occurred after 74 minutes. The molar ratio of AIBN toinhibitor was 24.

[0083] This comparative example shows that using a combination ofhydroquinone (p-dihydroxybenzene) and the AIBN inhibitor of theinvention (N,N′-di-sec-butyl-p-phenylenediamine) decreased the time toonset of turbity to about ½ of the time using AIBN alone.

Example 4

[0084] A mixture of 18.38 grams of acrylonitrile and 0.00241 grams ofDBPA inhibitor and 0.01862 grams of glacial acetic acid were placed inthe hot tube and 0.0492 grams of AIBN was then added to the mixture. Themixture was heated to 65° C. and the turbidity was monitored. The onsetof turbidity occurred after 130 minutes. The molar ratio of AIBN toinhibitor was 27. Acetic acid can be used in the process to control pHand this example demonstrates that acetic acid does not adversely impactthe performance of DBPA inhibitor.

Example 5

[0085] A mixture of 19.09 grams of acrylonitrile and 0.00244 grams ofDBPA inhibitor and 0.00156 grams of acrylic acid were placed in the hottube and 0.0515 grams of AIBN was then added to the mixture. The mixturewas heated to 65° C. and the turbidity was monitored. The onset ofturbidity occurred after 129 minutes. The molar ratio of AIBN toinhibitor was 28. Acrylic acid is a co-product that can be produced andthis example demonstrates that acrylic acid does not adversely impactthe performance of DBPA inhibitor.

Comparative Example E

[0086] This comparative example is to illustrate the use of an aminecompound which has two benzene rings, i.e., diphenylamine, to inhibitpolymerization. A mixture of 19.95 grams of acrylonitrile and 0.00184grams of diphenylamine (M.P. 52° C.) were placed in the hot tube and0.0363 grams of AIBN was then added to the mixture. The mixture washeated to 65° C. and the turbidity was monitored. The onset of turbidityoccurred after 84 minutes. The molar ratio of AIBN to inhibitor was 20.

[0087] Comparative Examples F & G These comparative examples are toillustrate the very poor results obtained in comparable use of compoundswhich are derivatives of hydroxyamine to inhibit polymerization.

Example F

[0088] A mixture of 20.46 grams of acrylonitrile and 0.00201 grams ofN,N-diethylhydroxylamine were placed in the hot tube and 0.040 grams ofAIBN was then added to the mixture. The mixture was heated to 65° C. andthe turbidity was monitored. The onset of turbidity occurred after 10minutes. The molar ratio of AIBN to inhibitor was 11.

Example G

[0089] Example 5 was repeated except that the molar ratio of AIBN toinhibitor was 4. The onset of turbidity occurred after 14 minutes.

[0090] For the purposes of the present invention, “fractionaldistillation” is defined as a method to separate a mixture of severalvolatile components of different boiling points; the mixture isdistilled at the lowest boiling point, and the distillate is collectedas one fraction until the temperature of the vapor rises showing thatthe composition of the vapor being distilled has changed: this vapor iscollected as a separate fraction. “Fractionation” is defined asseparation of a mixture in successive stages, each stage removing fromthe mixture some proportion of one of the substances as by distillation.Unless indicated otherwise, “column” is defined as an apparatus usedwidely for continuous separation of fluid (gaseous or liquid) componentsby vapor-liquid fractionation.

[0091] For the purposes of the present invention, “predominantly” isdefined as more than about fifty percent. “Substantially” is defined asoccurring with sufficient frequency or being present in such proportionsas to measurably affect macroscopic properties of an associated compoundor system. Where the frequency or proportion for such impact is notclear substantially is to be regarded as about twenty percent or more.The term “Essentially” is defined as absolutely except that smallvariations which have no more than a negligible effect on macroscopicqualities and final outcome are permitted, typically up to about onepercent.

[0092] Examples have been presented and hypotheses advanced herein inorder to better communicate certain facets of the invention. The scopeof the invention is determined solely by the scope of the appendedclaims.

That which is claimed is:
 1. A process for manufacturing an unsaturatedmononitrile which process comprises: reacting at least one feed compoundselected from the group consisting of propane, propylene, isobutane andisobutylene, with ammonia and a source of dioxygen in the presence of acatalyst a reactor to produce a reactor effluent containing thecorresponding unsaturated mononitrile; transferring the reactor effluentcontaining the unsaturated mononitrile to a quench/absorption sectionwherein the reactor effluent containing the unsaturated mononitrile iscontacted with at least a first aqueous stream to cool the reactoreffluent, and thereafter the cool effluent is contacted with at least asecond aqueous stream in an absorption column to separate and recoverthe unsaturated mononitrile in an aqueous solution; transferring theaqueous solution containing the unsaturated mononitrile to a recoveryand purification section where the unsaturated mononitrile is recoveredand purified in at least a first distillation column and a seconddistillation column; and maintaining within one or more of thedistillation columns an effective polymerization inhibiting amount of atleast one phenylenediamine compound represented by the formula:

wherein Ph is a phenylene group, and R₁ and R₂ are the same or differentand are hydroxyl free organic moieties having about 2 to about 10 carbonatoms with the proviso that neither R₁ nor R₂ is a phenyl group.
 2. Theprocess according to claim 1 wherein the effective polymerizationinhibiting amount is no less than about 5 parts per million parts ofunsaturated mononitrile present in the aqueous solution.
 3. The processaccording to claim 2 which further comprises admixing a liquid source ofthe phenylenediamine compound with an aqueous solution containing theunsaturated mononitrile which is being transferred into the firstdistillation column and/or second distillation column.
 4. The processaccording to claim 1 wherein the unsaturated mononitrile isacrylonitrile or methacrylonitrile, and the feed compound is acorresponding olefin selected from the group consisting of propylene andisobutylene.
 5. The process according to claim 1 wherein the organicmoieties R₁ and R₂ are members of a group consisting of ethyl, propyl,butyl, phenyl, hexyl, heptyl, octyl nonyl and decyl straight andbranched-chain hydrocarbon groups.
 6. The process according to claim 1wherein the phenylenediamine compounds are members of a group consistingof N,N′-dialkyl-p-phenylenediamine compounds.
 7. The process accordingto claim 1 wherein the unsaturated mononitrile is acrylonitrile, and thefeed compounds are members of a group consisting of propane andpropylene.
 8. The process according to claim 7 wherein thephenylenediamine compounds are members of a group consisting ofN,N′-di-sec-propyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine,N,N′-di-isobutyl-p-phenylenediamine, andN,N′-di-tert-butyl-p-phenylenediamine.
 9. The process according to claim8 wherein the compound is N,N′-di-sec-butyl-p-phenylenediamine.
 10. Aprocess for recovery of valuable nitrogen-containing organic compoundsformed by catalytic ammoxidation of propane, propylene or isobutylenewith ammonia and a gaseous source of dioxygen which process comprises:providing an aqueous solution comprising acrylonitrile ormethacrylonitrile, hydrogen cyanide and other organic co-products;fractionating the aqueous solution as by distillation in at least afirst multi-stage column and a second multi-stage column; andmaintaining within the columns an effective polymerization inhibitingamount of at least one compound represented by the formula:

wherein Ph is a phenylene group, and R₁ and R₂ are the same or differentand are hydroxyl free organic moieties having about 2 to about 10 carbonatoms with the proviso that neither R₁ nor R₂ is a phenyl group.
 11. Theprocess according to claim 10 wherein the effective polymerizationinhibiting amount is in a range upward from about 50 to about 1000 partsper million parts of unsaturated mononitrile present in the aqueoussolution.
 12. The process according to claim 11 wherein the organicmoieties R₁ and R₂ are members of a group consisting of ethyl, propyl,butyl, phenyl, hexyl, heptyl, octyl nonyl and decyl straight andbranched-chain hydrocarbon groups.
 13. The process according to claim 10wherein the phenylenediamine compounds are members of a group consistingof N,N′-dialkyl-p-phenylenediamine compounds.
 14. The process accordingto claim 13 wherein the unsaturated mononitrile is acrylonitrile, andthe feed compounds are members of a group consisting of propane andpropylene.
 15. The process according to claim 14 wherein thephenylenediamine compounds are members of a group consisting ofN,N′-di-sec-propyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine,N,N′-di-isobutyl-p-phenylenediamine, andN,N′-di-tert-butyl-p-phenylenediamine.
 16. The process according toclaim 14 wherein the compound is N,N′-di-sec-butyl-p-phenylenediamine.17. A process for recovery of valuable nitrogen-containing organiccompounds formed by catalytic ammoxidation of propylene and/or propanewith ammonia and a gaseous source of dioxygen which process comprises:providing an aqueous solution comprising acrylonitrile, hydrogen cyanideand other organic co-products of an ammoxidation reaction of propyleneand/or propane with ammonia and a gaseous source of dioxygen;fractionating the aqueous solution as by distillation in a multi-stagecolumn to obtain a high boiling fraction comprising a major amount ofthe acrylonitrile in the aqueous solution and a low boiling fractioncomprising a major amount of the hydrogen cyanide in the aqueoussolution; and maintaining within the column an effective polymerizationinhibiting amount of at least one phenylenediamine compound representedby the formula:

wherein Ph is a phenylene group, and R₁ and R₂ are the same or differentand are hydroxyl free organic moieties having about 2 to about 10 carbonatoms with the proviso that neither R₁ nor R₂ is a phenyl group.
 18. Theprocess according to claim 17 wherein the organic moieties R₁ and R₂ aremembers of a group consisting of ethyl, propyl, butyl, phenyl, hexyl,heptyl, octyl nonyl and decyl straight and branched-chain hydrocarbongroups. 19 The process according to claim 17 wherein thephenylenediamine compounds are members of a group consisting ofN,N′-di-sec-propyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine,N,N′-di-isobutyl-p-phenylenediamine, andN,N′-di-tert-butyl-p-phenylenediamine.
 20. The process according toclaim 19 wherein the compound is N,N′-di-sec-butyl-p-phenylenediamine.21. A process for recovery of valuable nitrogen-containing organiccompounds formed by catalytic ammoxidation of propylene or propane withammonia and a gaseous source of dioxygen which process comprises:providing an aqueous solution comprising acrylonitrile and high-boilingorganic compounds; fractionating the aqueous solution as by distillationin a multi-stage column to obtain a high boiling fraction comprising ahigh boiling fraction comprising essentially all the organic compoundsboiling above about 100° C., a sidedraw product stream comprising atleast 99 percent by weight of acrylonitrile, and a low boiling fractionsubstantially free of the high-boiling organic compounds; andmaintaining within the column an effective polymerization inhibitingamount of at least one compound represented by the formula:

wherein Ph is a phenylene group, and R₁ and R₂ are the same or differentand are hydroxyl free organic moieties having about 2 to about 10 carbonatoms with the proviso that neither R₁ nor R₂ is a phenyl group.
 22. Theprocess according to claim 21 wherein the organic moieties R₁ and R₂ aremembers of a group consisting of ethyl, propyl, butyl, phenyl, hexyl,heptyl, octyl nonyl and decyl straight and branched-chain hydrocarbongroups. 23 The process according to claim 21 wherein thephenylenediamine compounds are members of a group consisting ofN,N′-di-sec-propyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine,N,N′-di-isobutyl-p-phenylenediamine, andN,N′-di-tert-butyl-p-phenylenediamine.
 24. The process according toclaim 23 wherein the compound is N,N′-di-sec-butyl-p-phenylenediamine.25. A process for recovery of valuable nitrogen-containing organiccompounds formed by catalytic ammoxidation of isobutylene or isobutanewith ammonia and a gaseous source of dioxygen which process comprises:providing an aqueous solution comprising methacrylonitrile, hydrogencyanide and other organic co-products of an ammoxidation reaction ofisobutylene and/or isobutane with ammonia and a gaseous source ofdioxygen; fractionating the aqueous solution as by distillation in amulti-stage column to obtain a high boiling fraction comprising a majoramount of the methacrylonitrile in the aqueous solution and a lowboiling fraction comprising a major amount of the hydrogen cyanide inthe aqueous solution; and maintaining within the column an effectivepolymerization inhibiting amount of at least one compound represented bythe formula:

wherein Ph is a phenylene group, and R₁ and R₂ are the same or differentand are hydroxyl free organic moieties having about 2 to about 10 carbonatoms with the proviso that neither R₁ nor R₂ is a phenyl group. 26 Theprocess according to claim 25 wherein the phenylenediamine compounds aremembers of a group consisting of N,N′-di-sec-propyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine,N,N′-di-isobutyl-p-phenylenediamine, andN,N′-di-tert-butyl-p-phenylenediamine.
 27. The process according toclaim 26 wherein the compound is N,N′-di-sec-butyl-p-phenylenediamine.28. A process for recovery of valuable nitrogen-containing organiccompounds which process comprises: forming a gaseous reaction effluentby ammoxidation of propylene with ammonia and a gaseous source ofdioxygen in the presence of a heterogeneous catalyst comprising cobalt;contacting the gaseous reaction effluent with an aqueous liquid toobtain an aqueous solution comprising acrylonitrile, hydrogen cyanideand other organic co-products; fractionating the aqueous solution as bydistillation in a multi-stage column to obtain a high boiling fractioncomprising a major amount of the acrylonitrile in the aqueous solutionand a low boiling fraction comprising a major amount of the hydrogencyanide in the aqueous solution; and maintaining within the column aneffective polymerization inhibiting amount of at least one compoundrepresented by the formula:

wherein Ph is a phenylene group, and R₁ and R₂ are the same or differentand are hydroxyl free organic moieties having about 2 to about 10 carbonatoms with the proviso that neither R₁ nor R₂ is a phenyl group. 29 Theprocess according to claim 28 wherein the phenylenediamine compounds aremembers of a group consisting of N,N′-di-sec-propyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine,N,N′-di-isobutyl-p-phenylenediamine, andN,N′-di-tert-butyl-p-phenylenediamine.
 30. The process according toclaim 29 wherein the compound is N,N′-di-sec-butyl-p-phenylenediamine.